1. Field of the Invention
This invention relates to detonating cords with flash-suppressing coatings which prevent the detonated cord from igniting adjacent materials.
Detonating cord is an item of commerce consisting of a core of highly explosive material, usually particulate pentaerythritol tetranitrate (PETN), encased in various yarns and/or plastics. The cord is used to transmit a detonation front from one location to another, usually for the purpose of initiating one or several charges of high explosives. It functions by detonating when initiated by a blasting cap, the detonation being an intense chemical reaction of very short duration which liberates large amounts of heat and gases manifested in the form of a visible flash.
In use, such detonating cord is usually laid across the ground with subsidiary lines attached to main branch lines to initiate several charges of high explosives essentially at once. In some applications, the cord is laid over and through dry vegetation, for example when it is used in seismographic prospecting or in clearing fields. This has presented a particular problem, however, because the highly exothermic explosion of the PETN cord can easily, and has, initiated grass and forest fires.
The primary object of this invention is to provide a detonating cord with a coating which will inherently suppress the flash and thereby prevent the ignition of natural solid fuels with which the cord is in contact, without detrimentally affecting the velocity or energy of initiation of detonation and the ability of the cord to initiate the detonation of a high explosive charge.
2. Summary of the Prior Art
Most currently produced detonating cord consists of a PETN core encased in a polyethylene coating which prevents migration of water into the core which can render the core non-explosive. This coating, however, is generally inadequate to suppress the detonation flash and thereby prevent ignition of adjacent materials.
Some cords have been developed in an attempt to solve this problem. One such cord which has been used for clearing fire breaks in fighting forest fires comprises a PETN core, usually having 200 to 400 grains PETN per foot, encased in a layer of particulate chloride or phosphate salt sometimes including water of hydration. The salts are thought to absorb energy of the detonation, lower the detonation temperature and reduce the likelihood of igniting adjacent materials.
However, these detonating cords have several disadvantages. Special machinery is required to produce the cord, and the cord must be carefully checked over its entire length to insure that the salt is in place, a task complicated by the covering used to hold the salt in place. Furthermore, the amount of salt required exceeds the amount of explosive in the cord, so that the resulting bulk of the cord renders it difficult to tie the cords together to provide branched cords which transmit the detonating front to explosives in several locations.
Additionally, a detonating cord was developed but largely abandoned which utilized polyvinyl chloride (PVC), a flame retardant, as an attempted flash-suppressing coating. However, the use of PVC as a coating produced several problems. Aside from the fact that the PVC may have released vinyl chloride monomers which are though to be cancer causing agents, PVC itself could not be extrusion-coated on the PETN core to produce thin coatings at safe temperatures below about 320.degree. F. Coating compositions containing PVC diluted with plasticizers so as to be extrudable at safe temperatures were ineffective as flash-suppressants. Furthermore, the plasticizers tended to migrate into the PETN cord and reduce the detonation velocity of the cord at least in half, resulting in a cord with less than desirable initiating power.